The invention relates to a drive unit for driving the second driven axle of an all-wheel drive vehicle with a hydraulic coupling dependant on rotational-speed differences, comprising a driven case and two half shafts, which can each be connected to the latter by a friction clutch, the friction clutch being operatively connected to a hydraulic positive displacement machine that operates when a rotational-speed difference occurs between the case and the half shaft and has a first and a second rotor which can rotate in a case.
A coupling of this kind is known from WO Preliminary Published Application 95/23931, FIG. 14. In this, two mutually independent friction clutches are each provided in the case between a half shaft and the case and are each operatively connected to their own hydraulic positive displacement machine. The first rotor of one machine is connected to one half shaft for rotation in common, while the first rotor of the second machine is connected to the other half shaft. As a result, the torque transmitted to one half shaft rises with the rotational-speed difference between the respective half shaft and the case.
First of all, this has the disadvantage that the torque at the half shaft on the inside of the bend when cornering rises more sharply than that at the outside of the bend. However, this represents very unfavorable torque distribution and leads to stressing of the driveline, with higher wear on the tires and steering as a result.
The presence of two hydraulic positive displacement machines, which are never completely identical as regards characteristics and play (top and side gaps of the rotors) and diverge even more over their life due to differences in wear leads to uneven torque distribution, even during straight-ahead travel, to higher wear and disturbance to straight-line running. Moreover, the two hydraulic positive displacement machines are expensive to manufacture and take up a lot of installation space, which is very short between the half shafts, which should be as long as possible.
It is therefore the aim of the invention to propose a drive unit of the generic type which avoids these disadvantages; that is to say one which offers optimum torque distribution in combination with an inexpensive and small design. According to the invention, this is achieved by virtue of the fact that
a) the half shafts are connected to one another by a three-member averaging transmission, of which the first and the second member are each connected in terms of drive to one half shaft and the third member rotates at a rotational speed which is the average of the rotational speeds of the first and the second member,
b) a single hydraulic positive displacement machine is provided, the first rotor of which is connected in terms of drive to the third member of the averaging transmission and which is operatively connected to both friction clutches.
Thus only one hydraulic positive displacement machine is provided, and this is driven with the average of the rotational speeds of the two half shafts. As a result, both half shafts transmit approximately the same torque during cornering, and during straight-ahead travel the torque transmitted is not dependent on differences between two positive displacement machines. Both of these factors lead to lower wear and operation which is largely free of stress. Eliminating one positive displacement machine reduces the price and the space requirement of the unit, or one positive displacement machine can be given larger dimensions for a higher pump pressure, so that the friction clutches need fewer clutch plates.
In a preferred embodiment, the hydraulic positive displacement machine is arranged between the two friction clutches. This means that the connecting paths to both pistons acting on the friction clutches are equally short and, overall, that the case is very simple with many instances of symmetry and identical components.
There are a number of different options available for the averaging transmission (e.g. planetary transmission). It is preferably a bevel-gear differential, the differential gears of which are rotatably mounted in the third member. Its installation dimensions are then very small both in the axial and in the radial direction, especially since, of course, only the torque required to drive the positive displacement machine has to be transmitted, not the drive torque. At the same time, there is very wide latitude in the relative arrangement of the positive displacement machine and the averaging transmission.